Wood 2.0: mass timber and the tall buildings of tomorrow

An architect makes the case for the old being new again.

In a remarkable 240-page report simply entitled Tall Wood, architect Michael Green makes the case for wood as the ideal building material for skyscrapers and their structures. Green argues that it is imperative that society builds tall buildings out of wood rather than less-sustainable, more-polluting materials, and he claims that wood is a sorely underrated building material that is capable of much more than is typically asked of it.

His report (PDF) sets out a blueprint for the design of wooden structures up to 30 stories high, capable of performing in "high seismic areas" like his native Vancouver. Unusual for the construction industry, he's shared the report under a Creative Commons license. Ars looked into the report in detail and spoke to Arup structural consultant and wooden buildings advocate Hans-Erik Blomgren for another perspective.

A PR problem

In both the construction industry and the broader public, wood suffers from something of an image problem. Among the industry preconceptions tracked by Green are the notions that wood is both weaker and more expensive than concrete, that it is susceptible to fire (and that fire-protecting it is expensive), and that it will not withstand earthquakes. Ask Joe Public and one is liable to hear that wood rots, burns, shrinks, and gives off unpleasant gas, Green reports.

Such associations haven't materialized from the ether, Hans-Erik Blomgren argues. They come from people's direct experience with older wooden structures. "In commercial settings, some of the performance issues of older wood buildings such as poor acoustics—sound transmission and structure-borne noise between floors, creaking and bouncy floors—are something that building occupants won't tolerate in new build construction," Blomgren told Ars.

Timber, the massive kind

But both Green and Blomgren argue that the wooden building technologies available today are a far cry from the creaky wood-framed structures of the past. Crucially, Green's manifesto does not argue that the light-frame (or stud) construction commonly used in housing in the US should be applied to taller buildings. Instead, Green is pushing for the wider adoption of what is increasingly known as "mass timber" (or "massive timber"), a family of technologies that includes Cross Laminated Timber, Laminated Strand Lumber, and Laminated Veneer Lumber—products composed of thin layers of wood that are not only stronger than conventional timber, but thanks to their uniformity and straightness, are also simpler to design and build with.

Green calls his mass timber construction approach FFTT—a non-technical initialism that stands for Finding the Forest Through the Trees. "The acronym speaks to the idea that much of the sustainable building conversation is focusing on minutiae," Green writes in his report. "While even the minutiae contributes and is important, the big systemic change ideas are what we believe will be necessary for the built environment to tackle the scale of the climate change and housing demand challenges facing the world."

Old wood...

Building tall buildings out of wood isn't a new idea, and even without modern wood composites, wooden buildings have stood the test of time, even in areas of seismic activity. Green briefly points to the example set by Japanese pagodas built up to 19 stories tall which have proven adept at withstanding earthquakes and typhoons over centuries. The 122-foot (32.45-meter) tall pagoda at Huryu-ji was built during Japan's Asuka period (538 to 710) and is thought to be one of the oldest wooden buildings in the world. It is still standing today. A 1997 article in The Economist claims that only two or so of Japan's 500 pagodas have collapsed in 1,400 years. Some have even claimed that none have ever succumbed to earthquake or typhoon.

Blomgren looks to more recent history (as well as closer to home) for examples of wood's inherent soundness. "You might think I'd say it is not suitable as a building foundation; however, one of my favorite little known stories about the Brooklyn Bridge is that the caissons used to found the towers on bedrock down deep below the seabed are constructed out of vast quantities of untreated southern yellow pine timber," he told Ars. "These caissons are of course still in place at the bottom of the towers and functioning wonderfully. So yes, wood makes a fine foundation material in the right circumstance."

...New tricks

Ironically, Michael Green's FFTT proposal employs concrete foundations up to grade (or ground) level, though it's not required thereafter. Above the foundation, Green summarizes his design approach as "strong column—weak beam," with large panels of mass timber used to form the building's vertical structure, its lateral (sideways) load-resisting shear walls, and the floor slabs at each level. But the weak beam elements are not made from timber, and instead are steel beams designed to lend the structure ductility and added resilience when the wind blows or the ground shakes.

Solid-panel core and intersecting ductile steel link beams

Michael Green Architecture

"On a weight to strength ratio, engineered wood products generally match, and in some cases exceed, the performance of reinforced concrete," Green writes, before arguing that wood in mass timber form meets "basically all" desirable properties of a construction material from cost effectiveness and constructibility to durability, water ingress protection, and fire protection. Indeed, Green's report contains analyses of all these criteria and more, and sets out a peer reviewed theoretical model of how it could work. Green deliberately steers clear of suggesting how such buildings might look, instead offering up a structural design to which a variety of architectural facades might be applied.

When it comes to the economic and technical competitiveness of mass timber, Blomgren echoes Green. "Right out of the gate there is evidence that massive wood systems do compete on cost with concrete and steel," he said. "The reasonable expectation is that as the supply chain and number of manufacturers for massive wood systems grow, it will only become more cost competitive."

Big, but not that big

In his report, Green makes it clear that pilot projects are required to find and test the limitations of tall mass timber buildings. When the question of mass timber's limitations is put to Blomgren, he responds cautiously:

"It is hard to answer this question without having a small complex that it infers the dominant structural materials get a free pass here because they are perceived as having limitless capabilities," he said. "I'm inclined to say the biggest limitation of wood is society's perception of it. Yes, a psychological limitation. We have a hard time accepting that a material so pedestrian, familiar, and organic can do great things for us. If I'm an advocate for anything, it is simply that timber should have equal footing—a place at the table—with other structural construction materials."

That said, Blomgren is happy to discuss the technical challenges posed by the use of wood in building structures, including mass timber, though he claims most of these limitations can be overcome with good design.

Perhaps the most fundamental is that, above a certain size, wood's inherent strength and stiffness limitations make it more or less impossible to work with. But such uses are extreme, incorporating long-span highway suspension bridges, blast-resistant buildings, and super-tall skyscrapers: uses which Blomgren estimates constitute less than one percent of the built environment.

Blomgren also highlights wood's tendency to expand and contract as it absorbs and releases moisture: a dynamism that has to be considered during design. Wood can split, splinter and potentially fail completely where structural design has fallen short. And there's its tendency to decay due to fungal infection or insect infestation when misapplied to inhospitable environments or when not adequately treated (making the use of untreated southern yellow pine in the foundations of the Brooklyn Bridge all the more surprising).

From manifesto to portfolio

Though Green calls for an FFTT pilot project to more practically test the costs and construction implications of his principle, the more general use of mass timber is already on the rise. Both Green and Blomgren cite Waugh Thistleton's nine-story mass timber Stadthaus in London as an exemplar of mass timber construction, which employs Cross Laminated Timber walls exclusively, achieving a two-hour fire rating.

H8 in Bad Aibling, Germany

Shankula Architekten

Along similar lines are Sweden's eight-story, 134-apartment Limnologen (actually four buildings) in Vaxjo; the eight-story H8 in Bad Aibling, Germany (the mass timber structure was completed in three weeks); and the seven-story E3 in Berlin, which is fire-rated to 90 minutes.

Green argues much taller mass timber buildings are possible, yet construction of even these would be prohibited in British Columbia, and Blomgren's Seattle, due to local building code restrictions on so-called "combustible buildings." Such codes were largely written with light-frame construction rather than mass timber in mind. Russia limits such buildings to three stories, while the UK (perhaps surprisingly, given the conflagratory events of 1666) has no specified height limit.

Ultimately, both Green and Blomgren are adamant that mass timber is the future, but Blomgren argues that to get there will require quantifying the sustainable merits of wood and educating the construction industry and public about the high tech merits of mass timber. "Wood is not new, but the adhesives, glulam [glued laminated timber] presses, cutting equipment, carbide tipped screw fasteners, Digital documentation (BIM) to automated manufacturing, and prefabricated construction are all very state-of-the-art," he argues, before citing Metropol Parasol and the Centre Pompidou Metz as two Arup projects that, he suggests, will alter people's perceptions of the capabilities of timber construction.

On a more cautious note, Blomgren describes the delivery side of manufactured wood products as starved. "To get the ball rolling on this issue there needs to be some policy shifts and developer incentives created to open up a market," he told Ars. "Once this happens I’m quite certain that the manufacturing supply chain will grow."

The starved supply chain is not for a lack of timber. The mountain pine beetle outbreak seen in British Columbia and parts of the United State is effectively turning forests from CO2 stores into CO2 sources. These resultant emissions from dead tree decay could be reduced if the dead wood, which is perfectly serviceable for engineered wood products, was simply put to use.

Ultimately, Blomgren and Green agree that climate change should be one of the greatest motivational factors in a shift toward wooden buildings. "In a rapidly urbanizing world with an enormous demand to house and shelter billions of people in the upcoming decades, we must find solutions for our urban environments that have a lighter climate impact than today's incumbent major structural materials," Green writes.

Blomgren's emphasis is somewhat different. A healthy market for wood from sustainable sources increases the need for managed forests. "Wood is 49 percent carbon by dry weight, all of that carbon has been extracted from the atmosphere," he points out. "Properly managed forests can make a material difference in the carbon issue."

Whether mass timber will make the leap required to become, as Blomgren hopes, a building material on equal footing with traditional materials used for medium and larger buildings remains to be seen. To do so will, in many cases, require a policy shift. Rhetorically answering his own question of why we should build with wood, Green points to Canada's 397.3 million hectares of forest and woodland (10 percent of the world's forests), but points out that less than one percent of Canada's forests are harvested annually. "Economically, we suggest that the question of relevance should be 'why not wood?'"

I don't know enough about fire ratings to know if a 90 minute rating is okay or barely legal... but I'd be pretty scared to live in a high-rise building made out of wood without some serious assurance that it won't flash & burn. Very cool concept, though, and his research appears to be pretty exhaustive.

I would love to see a breakdown of how much waste is produced to make those mass timber sections. It is easy to sit back and claim that wood is sustainable without mentioning everything else that needs to be produced and exhausted to make massive columns of thin wooden planks glued(?) together.

I think this requires careful study, but I love the idea of building with wood.

Concrete is terribly energy-intensive to make, and its impacts are increased by the fact that making it emits large amounts of CO2 as part of the process (in addition to the CO2 emitted for combustion). In addition, reinforced concrete buildings don't age all that well, especially if poorly maintained. In this regard, they're even worse than solid concrete because the steel structure expands with humidity and causes cracks from the inside.

Wood, on the other hand, can be a good carbon sink, especially if used more intensively. Combine that with forest management and reforestation projects and you could even arguably have positive impacts on the environment.

P.S.: Am I the only one who can't shake those adolescent Catan jokes from their head after reading this article? Especially since they featured a bunch of them in a recent episode of The Big Bang Theory...

I think this requires careful study, but I love the idea of building with wood.

Concrete is terribly energy-intensive to make, and its impacts are increased by the fact that making it emits large amounts of CO2 as part of the process (in addition to the CO2 emitted for combustion). In addition, reinforced concrete buildings don't age all that well, especially if poorly maintained. In this regard, they're even worse than solid concrete because the steel structure expands with humidity and causes cracks from the inside.

Wood, on the other hand,

Wood will suffer from the same quality and cost cutting problems as any other building material. The corporate approach to dealing with construction will limit the useful lifetime of any building regardless of what kind of material it's made out of. It's foolish to think that simply changing the building material will really solve any current engineering or maintenance challenges.

Concrete can last for centuries. However, you first have to expect to be using the building in 100 or 500 years.

Re: wood as a carbon sink: I was wondering the same thing. Sure, once you've knocked down old growth, it stops sequestering carbon. The long term capabilities of an old growth tree vs it being used as building material- actually, I don't even know how to argue around that. Am I missing something?

Re: wood as a carbon sink: I was wondering the same thing. Sure, once you've knocked down old growth, it stops sequestering carbon. The long term capabilities of an old growth tree vs it being used as building material- actually, I don't even know how to argue around that. Am I missing something?

When you cut down the tree, you plant one its place. The new tree grows, all the while sequestering more carbon. Then you cut it down and plant a new tree in it's place. The new tree grows, all the while sequestering more carbon.. Repeat as necessary

Wood if treated with chemicals to change characteristics is fantastic way to build. It is lightweight, adding chemicals can make it flame resistant, or insect repellant, more pliable or many other properties that one can think up. The economies of scale do not have to be too large to make the price go down to very reasonable levels. We have the technology to make far superior wood than before and can improve upon it as technologies improve.

And as far as his "sustainability" argument, wouldn't cutting down massive amounts of "tall wood" trees increase CO2 content? Less trees means less CO2 absorbtion...

Managed forests mean that you have new trees growing constantly to replace the ones you cut down. It's basically a "factory" to pull CO2 out of the air and put it into buildings. The mass of wood that "lives" at the site of the forest stays constant while you constantly pull lumber (fixed CO2) out of it.

Personally, I'm much more worried that these buildings won't last more than about 100 years or so. Thomas Jefferson actually warned against building large or institutional buildings out of wood, because the architecture of our nation would be wiped clean every 100 years or so.

It seems like concrete is a sustainable building material already. We aren't running out and well made buildings should last for centuries. I would be interested in seeing a multi-century analysis of building materials, including brick, wood, concrete, metal, stone, etc.

My gut tells me that wood is not necessarily the most environmentally friendly or sustainable material source.

I think this requires careful study, but I love the idea of building with wood.

Concrete is terribly energy-intensive to make, and its impacts are increased by the fact that making it emits large amounts of CO2 as part of the process (in addition to the CO2 emitted for combustion). In addition, reinforced concrete buildings don't age all that well, especially if poorly maintained. In this regard, they're even worse than solid concrete because the steel structure expands with humidity and causes cracks from the inside.

Wood, on the other hand, can be a good carbon sink, especially if used more intensively. Combine that with forest management and reforestation projects and you could even arguably have positive impacts on the environment.

P.S.: Am I the only one who can't shake those adolescent Catan jokes from their head after reading this article? Especially since they featured a bunch of them in a recent episode of The Big Bang Theory...

I'd like to point out that fly ash (coal byproduct) used as a concrete additive reduces the carbon emissions of concrete. The recycling essentially puts the CO2 emissions at zero. It is also a good place to store the toxic material. There are questions about its suitability in use for residential areas though.

Also as a civil engineer, I enjoy the rare civil related articles on the site.

So chopping down trees is more environmentally friendly than making concrete? And by trees, I mean a LOT of trees.

I noticed during the article that there was a heavy emphasis on sustainable sources, and I think this is at the crux of the issue. This technology could be great if the lumber comes from sustainable tree farms, which is what the author seems to advocate for. If they don't use sustainable sources though, then yes it could be very bad. It's just like farming produce: it can be a very good thing for the environment or a very bad thing, with the key difference being whether or not it's sustainable. I could actually see buildings built using only sustainable lumber becoming a part of LEED or something similar, which would help promote responsible use.

Re: wood as a carbon sink: I was wondering the same thing. Sure, once you've knocked down old growth, it stops sequestering carbon. The long term capabilities of an old growth tree vs it being used as building material- actually, I don't even know how to argue around that. Am I missing something?

When you cut down the tree, you plant one its place. The new tree grows, all the while sequestering more carbon. Then you cut it down and plant a new tree in it's place. The new tree grows, all the while sequestering more carbon.. Repeat as necessary

And as far as his "sustainability" argument, wouldn't cutting down massive amounts of "tall wood" trees increase CO2 content? Less trees means less CO2 absorbtion...

Managed forests mean that you have new trees growing constantly to replace the ones you cut down. It's basically a "factory" to pull CO2 out of the air and put it into buildings. The mass of wood that "lives" at the site of the forest stays constant while you constantly pull lumber (fixed CO2) out of it.

Nice reply. I'm still wrestling with it. Basically, we should compare a forest full of young trees to a forest full of old trees. (Their is no carbon absorption happening when chopping down and using the wood for buildings.) Are young trees absorbing more carbon than old trees? Maybe they are.

sep332 wrote:

Personally, I'm much more worried that these buildings won't last more than about 100 years or so. Thomas Jefferson actually warned against building large or institutional buildings out of wood, because the architecture of our nation would be wiped clean every 100 years or so.

Thomas Jefferson was a great man, but I wouldn't use him as a construction materials expert.

And as far as his "sustainability" argument, wouldn't cutting down massive amounts of "tall wood" trees increase CO2 content? Less trees means less CO2 absorbtion...

That's where managed forests come in. Wood is a renewable resource.

Also, as the article points out, dead trees actually release CO2 back into the atmosphere, but are perfectly suitable for making engineered wood products. If you then plant replacements for all of them, you're reducing CO2 twofold.

I like the idea. Steel is incredibly energy intensive to extract, forge, and move and you can't grow more. I've lived in Europe in a wooden house that was built in 1772 (before the US Declaration of Independence) and it wasn't even the oldest building on the block. I've also been in plenty of cathedrals, castles, palaces, and skyscrapers. As a general rule, a building with wood, glass, sunlight, fountains, and gardens is more inviting than a building made of concrete, steel, and stone.

Re: wood as a carbon sink: I was wondering the same thing. Sure, once you've knocked down old growth, it stops sequestering carbon. The long term capabilities of an old growth tree vs it being used as building material- actually, I don't even know how to argue around that. Am I missing something?

When you cut down the tree, you plant one its place. The new tree grows, all the while sequestering more carbon. Then you cut it down and plant a new tree in it's place. The new tree grows, all the while sequestering more carbon.. Repeat as necessary

So, I guess we have trees that grow full size overnight?

You have several plots of land and you let them grow every year except the oldest that you harvest, you then replant it and move to the next plot the following year. Do you really think the lumber industries just cut everything down and keeping moving to where there are still trees? (in the USA at least)

And as far as his "sustainability" argument, wouldn't cutting down massive amounts of "tall wood" trees increase CO2 content? Less trees means less CO2 absorbtion...

Managed forests mean that you have new trees growing constantly to replace the ones you cut down. It's basically a "factory" to pull CO2 out of the air and put it into buildings. The mass of wood that "lives" at the site of the forest stays constant while you constantly pull lumber (fixed CO2) out of it.

Nice reply. I'm still wrestling with it. Basically, we should compare a forest full of young trees to a forest full of old trees. (Their is no carbon absorption happening when chopping down and using the wood for buildings.) Are young trees absorbing more carbon than old trees? Maybe they are.

sep332 wrote:

Personally, I'm much more worried that these buildings won't last more than about 100 years or so. Thomas Jefferson actually warned against building large or institutional buildings out of wood, because the architecture of our nation would be wiped clean every 100 years or so.

Thomas Jefferson was a great man, but I wouldn't use him as a construction materials expert.

It's a circular process when farming the trees. The CO2 absorption/emission is essentially the same every year.

making the use of untreated southern yellow pine in the foundations of the Brooklyn Bridge all the more surprising

If I understand correctly, the reason it works on the Brooklyn Bridge is that those cassions are "deep below the seabed" and therefore fully submerged all the time. It's repeated cycles of getting damp and drying out that makes wood rot, which is why wood is terrible at the groundline of a typical building.

Wood is a nice material but I'm not sure about using it for high buildings more on what I've learned about it's physical abilities, ie weakness/strengths... But using wood more in buildings like replacing plaster walls with wood and perhaps lower building sub 5 floors I guess I wouldn't mind. Sure fire "hazard" and so on yea but it might be worth to have a cool looking wood building with rather expensive fire-safety.. Wood "gathering" is something else witch needs some discussions, chopping down a forest an then replanting it, is, well not pretty. I know a few clear-felled areas that just got replanted and it won't be a nice forest again in less than 20 years plus it will be a rather odd one on that since how many "natural" forests has their trees in straight line...? :/

So chopping down trees is more environmentally friendly than making concrete? And by trees, I mean a LOT of trees.

I noticed during the article that there was a heavy emphasis on sustainable sources, and I think this is at the crux of the issue. This technology could be great if the lumber comes from sustainable tree farms, which is what the author seems to advocate for. If they don't use sustainable sources though, then yes it could be very bad. It's just like farming produce: it can be a very good thing for the environment or a very bad thing, with the key difference being whether or not it's sustainable. I could actually see buildings built using only sustainable lumber becoming a part of LEED or something similar, which would help promote responsible use.

I was thinking along the same lines. IF mass wood becomes a huge industry, can enough be harvested from sustainable forests to keep it environmentally friendly? And how will this new industry compete against the land demands of farming and livestocks.

And as far as his "sustainability" argument, wouldn't cutting down massive amounts of "tall wood" trees increase CO2 content? Less trees means less CO2 absorbtion...

Managed forests mean that you have new trees growing constantly to replace the ones you cut down. It's basically a "factory" to pull CO2 out of the air and put it into buildings. The mass of wood that "lives" at the site of the forest stays constant while you constantly pull lumber (fixed CO2) out of it.

Nice reply. I'm still wrestling with it. Basically, we should compare a forest full of young trees to a forest full of old trees. (Their is no carbon absorption happening when chopping down and using the wood for buildings.) Are young trees absorbing more carbon than old trees? Maybe they are.

It doesn't matter, as long as they grow back to full size before you cut them, the rate of CO2 fixation will be steady.

So chopping down trees is more environmentally friendly than making concrete? And by trees, I mean a LOT of trees.

I noticed during the article that there was a heavy emphasis on sustainable sources, and I think this is at the crux of the issue. This technology could be great if the lumber comes from sustainable tree farms, which is what the author seems to advocate for. If they don't use sustainable sources though, then yes it could be very bad.

What I'm worried about is if people think "sustainable forests" are replacements for actual (non-harvested) forests. Wildlife generally doesn't live in managed forests because the noise drives them away and the habitat gets razed every few years. It's not enough to have enough sustainable forests, you also need enough "normal", undisturbed forests to keep the environment from going bye-bye.

I don't know if anyone has said anything like this, but I can see a few issues like fire (I mean, won't a wooden struture make the building caught fire faster and collapse sooner?) and termites. Also, won't humidity cause problems to the structure?

Personally, I'm much more worried that these buildings won't last more than about 100 years or so. Thomas Jefferson actually warned against building large or institutional buildings out of wood, because the architecture of our nation would be wiped clean every 100 years or so.

Thomas Jefferson was a great man, but I wouldn't use him as a construction materials expert.

Why not? He was a real polymath and spent a huge amount of time thinking about architecture, and specifically what the architecture of America would be. His buildings are iconic and highly influential in American architecture. https://en.wikipedia.org/wiki/Jeffersonian_architecture